Preparation of cellular polyolefins



United States Patent Oflice Re. 26,850 Reissued Apr. 7, 1970 PREPARATIONOF CELLULAR POLYOLEFINS David A. Palmer, Wilmington, Del., assignor toHercules Incorporated, a corporation of Delaware No Drawing. OriginalNo. 3,341,481, dated Sept. 12,

1967, Ser. No. 380,068, July 2, 1964. Application for reissue Feb. 4,1969, Ser. No. 807,459

Int. Cl. C08d 13/08; C08i 1/14 U.S. C]. 260-2.5 28 Claims Matterenclosed in heavy brackets appears in the original patent but forms nopart of this reissue specification; matter printed in italics indicatesthe additions made by reissue.

ABSTRACT OF THE DISCLOSURE This invention involves a process ofpreparing cellular thermoplastic polymers which comprises heating in aclosed mold the thermoplastic polymer, an azr'do crosslinking agent, anda blowing agent to expand and foam said polymer.

This application is a continuation-in-part of my application, U.S.Serial No. 304,977, filed August 27, 1963 and now abandoned.

This invention relates to a process of preparing cellular products fromthermoplastic polymers and more particularly to a process of preparing athermoplastic polymer foam of fine celled, uniform structure and lowdensity.

Many processes have been used for the preparation of articles fromfoamed polymers, particularly of polystyrene. One of the most commonprocedures is that wherein beads, granules, etc., of a polymer, such aspolystyrene, containing a volatile liquid blowing agent arepre-expanded, then placed in the mold and heated with steam to furtherexpand the beads, fuse them together and fill the mold. This procedurehas many disadvantages and in fact is not adaptable to, for example, thehigher melting polypropylene, because of the difficulty of welding theparticles together without collapse of the foam. Consequently, when sucha process is used, the welds are often imperfect and also are subject tofracture at the weld seam. Another process that has been used is thethermoforming of an extruded, foamed sheet of polystyrene, but thismethod is of greatly limited use due to the fact that the cold, foamedsheet must be inch or less in thickness because of the critical heatingcycle that is required.

Now in accordance with this invention, it has been found that cellularthermoplastic polymers can readily be produced by heating underpressure, as for example in a closed mold, a composition comprising thethermoplastic polymer, a blowing agent, and an azido cross-linkingagent, to a temperature above the softening point of the composition andsufficient to release the gas from the blowing agent and to effect theazido modification of the polymer, releasing the pressure, as by openingthe mold, allowing the composition to expand in three dimensions, andthen while still hot, thermoforming or otherwise shaping the foam intothe desired shape.

The process of this invention makes it possible to p epare and, ifdesired, thermoform thick sheets of foam on rapid cycles, thusovercoming both the thickness limitation and the critical heating cyclesrequired with other materials. The present process permits rapid anduniform heating by keeping the foamable composition under compressionduring the heating cycle, since, under compression, foaming cannot takeplace and, accordingly, heat transfer rates are uniform and high.Furthermore, more uniform foams are obtained since foaming is suppresseduntil the composition is uniformly heated. When the pressure isreleased, expansion takes place, which expansion is surprisingly athreedimensional expansion, the foam retaining the shape of the mold.Because the azido modification of the polymer is effected iust prior toor simultaneously with expansion, in the process of this invention, thefoam has adequate stability to permit its being easily handled withoutcollapsing and the foam can be transferred to a forming or moldingdevice and formed while it is still hot or cooled between parallelplates to yield a flat sheet, etc.

Any thermoplastic polymer can be foamed and shaped by the process ofthis invention. Exemplary of the thermolastic polymers which can be sofoamed are polyolefins such as high and low density polyethylene,stereoregular polypropylene, ethylene-propylene copolymers, particularlythe at least partially crystalline polymers which generally contain atleast about ethylene, ethylene-vinyl acetate copolymers,ethylene-acrylate copolymers, polystyrene,acrylonitrile-butadiene-styrene terpolymers, and blends of any of thesepolymers, polyvinyl chloride, both rigid and plasticized, etc. For manyapplications, it is desirable to increase the flexibility of the finalarticle by adding an elastomeric hydrocarbon polymer to the blend ofthermoplastic polymer, cross-linking agent, and blowing agent, and thenprocessing by the invention. The amount of the elastomeric polyolefinincorporated can vary over a wide range, but generally will be about 50%or less by weight of the thermoplastic polymer. Exemplary of theelastomeric hydrocarbon polymers that can be so incorporated arepolyisobutylene, butyl rubber (a polyisobutylene which contains arelatively small amount of unsaturation), ethylene-propylene copolymerrubbers, (i.e., amorphous copolymers), ethylene-propylene-dieneterpolymer rubbers, etc. Exemplary of the dienes in the latterterpolymers are dicyclopentadiene, butadiene, isoprene, norbornene,S-methyl-Z-norbornene, 1,4-hexadiene, 6-methyl-l,5-heptadiene, etc.

Any of the well-known chemical blowing agents can be used in thepreparation of the foamed articles in accordance with this invention as,for example, azobis(formamide), diazoaminobenzene,N,N'-dinitrosopentamethylene tetramine,N,N'-dirnethyl-N,N'-dinitrosoterephthalamide, p,p-oxy-bis(benzenesulfonyl semi-carbazide), azobisfisobutyronitrile), p,p'-oxy-bis(benzenesulfonyl hydrazide), p,p-diphenyl-bis(sulfonyl hydrazide),benzenesulfonyl hydrazide, m-benzenebis(sulfonyl hydrazide), etc. Any ofthe well-known solvent blowing agents may also be used in this inventionas, for example, methyl chloride, methylene chloride,monochlorotrifluoromethane, mouochlorodifiuoromethane,dichlorotetrafiuoroethylene, trichloroethylene, chloroform, carbontetrachloride, and low boiling hydrocarbons such as butane, pentane,hexane, etc. Accordingly, any compound which decomposes or volatilizesto yield at least one mole of gas per mole of blowing agent at atemperature of 190 C. or less may be used.

Any azido cross-linking agent can be used in the preparation of thefoamed articles in accordance with this invention. Thus, anypoly(sulfonazide), i.e., any compound having the general formula L 02 at where R is an organic radical inert to the cross-linking reaction andx is an integer greater than 1, can be used in the process of thisinvention. Preferably, x will be an integer from 2 to and R will beselected from the group of organic radicals consisting of alkylene,arylene, aralkylene, and alkarylene radicals; however, these radi calscan also contain ether, alcohol, halogen, etc., groups which are inertto the cross-linking reaction. Exemplary of the poly(sulfonazide)s thatmay be used are 1,7- heptane-bis(sulfonazide), 1,10decane-bis(sulfonazide),

1.1l-undecane-bis(sulfonazide'), 1,12 dodecane bis(sulfonazide),7-oxa-tridecane-l,l3-bis(sulfonazide), 6-thiaundecane 1,11bis(sulfonazide); chloroaliphatic poly (sulfonazide)s such as thepoly(sulfonazide) produced from a chloroand sulfochlorinated mixture ofpetroleum hydrocarbons containing at least one chlorine atom and atleast two sulfonazide groups per molecule;1,9,lS-octadecane-tris(sulfonazide) poly(ethylene sulfonazide), poly(sulfonazido-methyl styrene), l.3- and l,4-bis(sulfonazidomethylbenzene), 1,3-benzene bis(sulfonazide), l-octyl- 2.4.6-benzenetris(sulfonazide). 4,4'-diphenylmethane bis (sulfonazide), 4,4-diphenylether bis(sulfonazide), 4,4- bis octadecylbiphenyl-3,5,3',5'-tetra(sulfonazide), 4,4- diphenyl disulfidebis(sulfonazide), 1,6 bis(4'-sulfonazidophenyl) hexane, 2,7-naphthalenebis(sulfonazide), etc. Another class of azido cross-linking agents thatcan be used are azidoformates which have the general formula where x isat least 1, preferably from about 1 to about 100, and R is an organicradical, inert to cross-linking reactions, containing at least onecarbon atom per azidoformate group. Exemplary of these azidoformates arethe alltyl azidoformates such as n-octadecyl azidoformate,tetramethylene bis(azidoformate), pentamethylene bis (azidoformate); thecyclic alkyl azidoformates such as 2- (l-p-methenyl-S-oxy) ethylazidoformate; the aromatic azidoformate such as phenyl azidoformate,a,n'-p xylylene-bis(azidoformate), 2,2-isopropylidene-bis[p,p-phenylazidoformate); the azidoformate ethers such as2,2'-oxydiethyl-bis(azidoformate), 2,2 oxydipropyl bis(azidoformate),2,2 ethylenedioxydiethyl-bis(azidoformate), the tetraazidoformate ofpentaerythritol-propylene oxide adduct, the azidoformate thioethers suchas 2,2-thiodiethyl-bis(azidoformate), 4,4'thiodibutyl-bis(azidoformate);etc. Still another class of azido cross-linking agents that can be usedare the aromatic poly-azides having the general formula R(N where R isan aromatic grouping inert to the cross-linking reaction, and x is aninteger greater than 1. Preferably x will be an integer from 2 to 200and R will be selected from the group of organic radicals consisting ofarylene and alkarylene radicals. Exemplary of the aromatic polyazidesuseful in this invention are m-phenylene diazide,2,4,6-triazido-benzene, 4.4'-diphenyl diazide, 4,4'-diphenylmethanediazide, 4,4- diazido diphenyl amine, 4,4'-diazido diphenylsulfone, 2,7-diazidonaphthalene and 2,6-diazido-anthraquinone. Thus, any compoundhaving at least one azido group in the molecule and preferably two ormore can be used as the azido cross-linking agent to prepare the foamsin accordance with this invention.

The amount of the azido cross-linking agent utilized can be varied overa wide range and will depend upon the cross-linking agent and thechemical blowing agent may type of azido cross-linking agent used, thepolymer being foamed, the properties desired in the final foam, etc. Itmust be an amount that is sufficient to prevent rupture of the cellwalls when the foaming action takes place and at the same time preventsagging of the hot foamed sheet so that it can be processed as forexample, thermoformed, etc. Generally, it will be an amount of fromabout 0.01% up to about or more, preferably from about 0.1% to about 5%and more preferably from about 0.1% to about 2% by weight of thepolymer. Depending upon the amount and type of azido cross-linking agentused, actual cross-linking of the polymer may or may not occur. Thus,for example, at low levels of polysulfonazides, particularly those ofhigh molecular weight, there will be an insufficient amount to achieveactual crosslinking (insolubilization of the polymer), yet the polymerwill be modified to such an extent that collapse of the cell walls isprevented, which collapse would occur if no azido cross-linking agentwere used. Larger amounts of the azido cro s-linking g nt will be usedwhen a crosslinked, foamed product is desired. The amount of blowingagent incorporated will obviously depend upon the degree of blowingdesired; that is, the density desired for the final foamed product andthe types of blowing agent used. The process of this invention is ofparticular importance in the production of articles of foamedpolyolefins.

As pointed out above, the composition which is heated to produce thecellular thermoplastic polymer which can then be thermoformed orotherwise shaped, is a mixture or blend of the polymer, azidocross-linking agent and blowing agent. Any desired means can be used inmixing or blending these components. In the case of the chemical blowingagents, the azido cross-linking agent and the blowing agent may be mixedinto a diluent such as ace tone, which may also contain a stabilizer orother modifier for the polymer, and the polymer in finely divided formmay then be added and mixed into a slurry. On evaporation of the diluentan intimate mixture of the polymer, cross-linking agent and blowingagent is obtained which may then be used directly in the mold. The azidocross-linking agent and the chemical blowing agent may also be blendedin dry form with the powdered polymer by means of a high-speed mixersuch as a Waring Blendor or Henschel mill. This dry mix may then be useddirectly in the mold. When solvent types of blowing agents are employed,the polymer and azido cross-linking agent can be blended by any desiredmeans and formed into pellets, strips, etc., which can then be soaked inthe volatile blowing agent and used. Obviously, many other variationscan be made in the method of blending or mixing this threecomponentcomposition. In addition, additives, such as light and heat stabilizers,dyestuffs and pigments, flame retardants, including organic andinorganic flame retardants, such as chlorinated paraffin wax, antimonyoxide and other such materials, cell nucleating agents, etc., can beincorporated in the composition that is foamed.

The temperature and length of the heating cycle will depend on thethermoplastic polymer or elastomer blend thereof being foamed, the azidocross-linking agent used, and the blowing agent used. In general, itwill be a temperature above the softening point of the composition andsufiicient to release the gas from the blowing agent and to effect theazido modification of the polymer. The softening temperature of theblend will depend upon whether the blowing agent is a solvent type or achemical type. Thus, with a solvent type blowing agent, the polymer willat least partially dissolve so that the softening temperature of theblend can be as low as C. On the other hand, with a chemical blowingagent, the softening temperature of the blend will be the melting pointof the polymer. Accordingly, the cross-linking and blowing temperaturewill be from the softening temperature up to about 275 C. The period oftime required will depend on the temperature used, the specimenthickness, etc., but usually will be about 1 to about 15 minutes.

The expansion of the expandable composition which takes place onreducing the pressure after the heating cycle may be carried out in asingle or multiple stage pressure reduction process. Thus, if thecomposition is heated in a mold under pressure, expansion takes place inall three dimensions when the mold is opened and the cellular productwill have the same general shape and configuration as the mold in whichthe expandable composition was heated. This cellular product can then becooled in that shape or it can be thermoformed, while still hot, intoany desired shape. As pointed out above, the polymer is modified by theazido cross-linking agent in this process, just prior to orsimultaneously with the expansion, hence if the foam is to bethermoformed, this operation can be carried out immediately and prior toany substantial cooling of the foam. In the same way, if the reductionof pressure, after the heating cycle is carried out in more than onestatge, the time lapse and drop in temperature is preferably held at aminimum. The heating cycle can be carried out in a compression mold,extruder, or other means. In the case of the mold, the pressure isreduced by the sudden opening of the mold. In the case of an extruder,the composition heated in the extruder to the desired temperature can beextruded onto a belt at atmospheric pressure to allow expansion.

There are many advantages in carrying out the reduction in pressure onthe heated expandable composition in multiple stages. A much smallermold can be used for the compression heating cycle than could otherwisebe used for a given density of foam. Also, for a given size mold,smaller amounts of blowing agent are required for a given density offoam. This reduction in pressure in stages is readily carried out byfirst reducing the pressure after the heating cycle to atmosphericpressure and, then while still hot, further reducing the pressure tosubatmospheric pressure by applying a vacuum. For example, the mixtureof polymer, azido cross-linking agent and blowing agent can be heated ina closed mold under pressure and after the desired temperature isreached, suddenly releasing the pressure by opening the mold. The foam,which has expanded in all three dimensions, can then be immediatelyplaced in a vacuum chamber and the pressure further reduced to furtherexpand the foam. In this second stage of expansion, the foam can at thesame time be shaped to the desired foam. The latter operation can beachieved by placing the foam, after the first stage of expansion, into awire cage, cloth sack, or other shaping container of the desired form,or between platens, etc., and then placing the assembly in a vacuumchamber and reducing the pressure until the foam fills the container, orotherwise assumes the desired shape, after which it is cooled.Obviously, the expansion in this second stage can be operated as a one-,two-, or three-dimensional expansion, as desired, to produce the typeand shape wanted in the final cellular product. Hence, the operation isvery flexible and enables the production of articles having essentiallythe same cell size throughout, or small cell size in one portion withlarger cell size in another, longer cells in one direction, etc. Manyother variations will be apparent to those skilled in the art.

Foamed articles produced by the process of this invention have tough,smooth, closed surface skins which may, if desired, be backed by a layerof cells slightly larger than those at the center of the foam. Thislayer increases the stiffness of the skin. The depth of the coarse foamlayer can be increased by increasing the heating time in the press. Incases where the coarse cell layer is undesirable, i.e., where a lessstiff skin is desired, the layer can be reduced by shortening theheating time. It can be completely eliminated and fine cells obtained inthis layer by including a fiinely divided pigment as a nucleating agent.The process of this invention is particularly outstanding for theproduction of foams with fine, uniform,

closed cells, the cells of which foams will vary from less than 0.001inch to 0.025 inch.

The following examples will illustrate the process of this invention.All parts and percentages are by weight unless otherwise indicated.

Examples 1-3 In Example 1 and the control, preforms of the polymer,azido cross-linking agent (DSA, i.e. decamethylene disulfonylazide) andblowing agent compositions were formed by adding to 100 parts ofacetone, 0.20 part of the stabilizer known as Santonox[4,4'-thiobis(fi-tert-butylm-cresol)], varying concentrations of theazido crosslinking agent and varying concentrations of azobis(formamide)as blowing agent. These acetone mixtures were vigorously stirred, pouredover 100 parts of the finely divided polypropylene and mixed into aslurry. With occasional stirring the solvent was evaporated, leaving anintimate mixture of polymer, stabilizer, cross-linking agent, andblowing agent. The mixtures were each then placed on a 6 x 16 inch,two-roll mill at 168 C. and compounded for 7-10 minutes with repeatedcross-cutting and end-rolling. When thoroughly milled, the hot plasticwas removed from the mill in the form of a /a inch thick sheet. Themilled sheets were cut into sizes to yield the weight required to fillpreform picture-frame molds of definite dimensions. Preforms wereprepared by placing the cut pieces in these molds at 177 C. for 4minutes without pressure, followed by 2 minutes at 600 p.s.i. They werethen cooled under pressure.

In Examples 2 and 3, the blend of polypropylene, elastomer, azidocross-linking agent (DSA) and azobis(formamide) as blowing agent wascompounded directly on a two-roll mill Without the previous solventslurry step, removed from the mill, and cooled with Dry Ice, choppedinto pellets and then placed in the mold without molding into a preform.

Compounded pellets or preformed sheets, cut to the mold dimensions shownin Table I, were placed in the mold between chromed platens and theentire assembly was put in a heated hydraulic press, which was thentightly closed. At the completion of the heating time, the press wasopened rapidly, permitting the foam to expand out of the mold.

Tabulated below is the polymer or blend of polymers used, the amount ofblowing agent and cross-linking agent, mold dimensions and temperatureand heating period, along with the size of the foamed sheet produced,density of the foam and description thereof.

The expanded sheets possessed excellent form stability at the foamingtemperature in every case, in sharp contrast to the control where anattempt was made to produce a polypropylene foam under identicalconditions but without incorporating a cross-linking agent. After forma-TABLE I Cross-Linking Press Conditions Parts Agent; Example Azobis- MoldDimensions Density Number Thermoplastic form- Dimensions of Foam Results(Hr/c it.)

amide (inches) Temp. Heating (inches) Type Parts C.) Time (min) Contr0lPrglgpropylene (RSV 3.0 0 4 x 4 X V 235 1% 1 do 3. 0 DSA 0. 4 x 4 x 2351% 6 x 6 x V; Fine cells 0.00l to 7, 5

0.008". Very uniform structure. 2 Poly ropylene (RSV 5. 0 BSA 1.5 6 x 6x yg 225 1 10 x 10 x Fine cells 0.001 to 4. 5

3.2?75%, Ethylene- .005. Very uni- Propylene Rubber form structure. (RSV2.1) 25%, Propylene32 mole percent. 3 Poly ropylene (RSV 5.0 DSA 1.5fixfixl 225 1 10x10x% do 5 0 3.2l 75%, Polytecbutylene 25%.

I No foam obtained. Material in mold was a sticky mass.DSA-Decarnethylene disulionylazide.

tion of the foams, they were removed from the press and placed betweencooling platens at a pressure of l to 2 [3.5.1.

The abbrevation RSV in these and the following examples is used todenote the reduced specific viscosity of the polymer as measured on a0.1% solution of the polymer in decahydronaphthalene at 135 C.

Examples 46 Examples 1-3 were exactly repeated except that after eachfoam sheet was prepared, it was immediately taken out of the press andplaced between matching metal cup molds, thermoformed by application ofpressure, and then permitted to cool. In each case the foam conformedexactly to the mold cavity, producing a fine-celled, foamed plastic cup.This operation was not possible in the polyproylene control example,since without the cross-linking agent, the sheet was too sticky and softto handle.

Example 7 A mixture of 100 parts of stereoregular polypropylene havingan RSV of 4.2, 0.2 part of Santonox and 0.05 part of calcium silicate(as a cell nucleator) were dry blended and then extruded at 216 C. andthe extrudate was chopped into /s-inch molding pellets. The moldingpellets were soaked, at 35 C., in a mixture of 0.2 part oftetramethylene bis azidoformate and 4.8 parts of methylene chlorideuntil a weight gain of 6% was observed. The pellets were then exposed toair until their surfaces were dry. Sixty parts of these pellets werethen placed in a 6" x 6" x Vs" mold which was closed tightly in ahydraulic press heated to 160 C. After a heating period of 3 minutes,the press was opened permitting the foam to expand out of the mold.After cooling, the foam was found to have a density of 10 lbs/cu. ft.and a uniform cell structure, all cell dimensions of which were 0.030 to0.090 inch.

When this example was repeated, except that the tetramethylene bisazidoformate was omitted, only a sticky lbs/cu. ft., was immediatelyplaced in a vacum chamber and the pressure was reduced to 7 inches ofmercury, whereby the foam expanded further. The density of the finalfoam 50 produced was 2.5 lbs/cu. ft.

In Example 10, the process of Example 9 was repeated except that thefoam obtained on opening the mold was immediately placed in a wire meshrestraining cage, which was then placed in the vacum chamber and thefoam was expanded to the confines of the cage when the pressure wasreduced. The shaped foam article so obtained had a density of 3.8lbs/cu. ft.

Examples 11 and 12 Examples 8 and 9 were repeated except that thepreforms were prepared from a composition made up of 100 parts ofpolypropylene (RSV of 3.2), 0.5 part of decamethylene disulfonylazide,and 0.20 part of azobis- (formamide).

The foam produced in Example 11 on opening the mold and cooling betweenplatens as described in Examples 1 and 8 had a density of 41 lbs/cu. ft.

The foam produced in Example 12 was expanded in the first reduction ofpressure to a density of about 40 lbs/cu. ft. and when furtherimmediately expanded under vacuum (while still hot) had a final densityof 14 lbs/cu. ft.

Examples 13-24 In each of these examples a mixture of 100 parts of thethermoplastic polymer, or blend thereof, a given amount of the blowingagent azobis(formamide), and 0.5 part of Santonox was two-roll milledfor 8 minutes at 170 C., at which time a given amount of the azidocross-linking agent was added on the mill and the mixture was thoroughlyworked for 3.5 minutes. The mixture was then removed and allowed to coolto room temperature. An amount of the mixture, calculated to fill 90% ofthe volume of a 5 x 5 x A inch mold, was placed in the mold andsubjected to 1200 p.s.i. and 215 C. for 4.5 minutes. The

mass was obtained instead of a {02mm 4') press was then opened tosuddenly release the pressure and the material expanded to yield thefoam. Examples 8-10 Tabulated below is the polymer or blend of polymersused, the amount of blowing agent and cross-linking Preformed sheetswere prepared by the procedure deagent, and the properties of the foamproduced. The scribed in Example 1 using 0.5 part of decamethylenedicross-linking agents are designated in the table as follows:sulfonylazide and 2.3 parts of azobis(formamide) per DSA-Decamethylenedisulfonylazide; DDSADodeca- 100 parts of polypropylene (RSV of 3.2).methylene disulfonylazide; DPSA4,4-diphenyl di- In Example 8 thepreformed sheet was placed in a sulfonylazide; KSA-achloropoly(sulfonazide) prepared mold and heated under pressure exactlyas described in from a commercial mixture of hydrocarbons having 11-Example 1. The foam so prepared had a density of 9.8 12 carbon atoms permolecule and containing an average lbs/cu. ft. 50 of 8% chlorine and 2sulfonazide groups per molecule.

TABLE II Cross-Linking Agent Example Thermoplastic Parts Azobls-Dimensions of Density fonnamide Foam (inches) (lbJcu. ft.)

Type Parts Control Polyethylene (Density 0.962) 6 No foam produced. 13.do 6 BSA 0.5 11x1lx0.52 14 ..do 2.8 DSA 0.5 0x9x0.48 15 Polyethylene(Density 0.045) 6 DSA 0. 5 11 is 11x 0.50 5 16 Polyethylene (Density0.962) 6 DDSA 0. 75 10.8 X 10.8 x 0.50.. 5.5 11 .do 6 DPSA 0.5 10x10x0.57 1s .do 2.8 KSA 0. 75 8.5x 8.51: 0.45 10 1s Polyethylene 75% (Density0.962), Poly- 6 DSA 0.5 10.5x 10.5x0.5-. s

propylene (RSV 3.2). 20 Polyethylene Polypropylene 50 6 DSA 0. 5 10.5 x10.5 x 0.5. 6 Polyethylene 25%, Polypropylene 6 DSA 0. 5 10.51: 10.5 x0.5.... 6 J2. Polyethylene 75% (Density 0.062), Poly- 6 DSA 0.510.5x10.5x 0.5.... 6

isobutylcno 25%. 23 Polystyrene (mol. wt. 150,000) 2 DPSA 0. 5 9 x 0 x0.44 11 .24 Ethylenc-propylene Copolyrncr (9 mole 3 DSA 0.5 10x 10x 0.487. 5

percent propylene; RSV 2.1).

The process of Example 8 was exactly duplicated in Example 9 except thatinstead of cooling the foam between cooling platens at a pressure of 1to 2 p.s.i. as was done there, the foam, produced on releasing thepressure by opening the mold and having a density of above 10 In eachcase the foam had a fine uniform, closed cell structure, the cell sizeof the foams in Examples 13-23 being I-20 mils and the cells of the foamof Example 24 being 5l5 mils in size. The foams produced in Examples19-21 had increased hardness and stiffness as the concentration ofpolypropylene was increased, the foam of Example 19 having a compressivestrength of 49 p.s.i. and modulus of 1800 p.s.i., the foam of Example 20having a compressive strength of 64 p.s.i. and modulus of 2700 p.s.i.,and that of Example 21 having a compressive strength of 75 p.s.i. andmodulus of 2800 p.s.i. The foam produced in Example 22 was more elasticthan the foam produced from polyethylene alone (Example 13) and had atensile modulus of 4300 p.s.i.

Example 25 A mixture of 100 parts of poly(vinyl chloride), parts ofazobis(formarnide), 6 parts of dibasic lead phthalate, 10 parts ofepoxidizted soybean oil (mol. wt. about 1000) as plasticizer, and 0.5part of decamethylene disulfonylazide were worked on a two-roll mill for6 minutes at 170 C. The mixture was removed and after allowing to coolto room temperature, 90 parts of the mixture was placed in a 6 x 6 x A;inch mold under about 1200 p.s.i. pressure for 4 minutes at 215 C. Thepress was then opened to suddenly release the pressure. The yellowfoamed product was then cold formed to A inch thickness. The foamedsheet so produced had a fine uniform cell structure and a density of 18lbs./cu. ft.

As may be seen from the foregoing examples, the process of thisinvention makes it possible to produce thermoplastic cellular articlesby a process that is not only commercially feasible but also iseconomical. Very rapid foaming and thermoforming cycles are possible andonly small presses are required since the finished foam sheet or othershaped article is much larger than the press capacity, etc.

The process of this invention can be used in the manufacture of any flator molded foam article as, for example, wall panels, buoyant devices,shipping crates and boxes, insulated containers, padded seating,resilient headliners and door panels for automobiles, etc. Designs,printing, etc., can be pressed into the smooth skin of the foam andarticles can be prepared having high density sections and low densitysections all in a single operation. Many other variations can obviouslybe made in the preparation of the final cellular product.

What I claim and desire to protect by Letters Patent is:

l. A process for preparing cellular thermoplastic polymers whichcomprises (1) heating in a closed mold a preformed compositioncomprising (a) a thermoplastic polymer selected from the groupconsisting of polyethylene, polypropylene, ethylene-propylenecopolymers, ethylene-vinyl acetate copolymers, ethylene-alkyl acrylatecopolymers, polystyrene, acrylonitrile-butadienestyrene terpolymers, andpolyvinyl chloride,

(b) an azido crow-linking agent which decomposes at a temperature withinthe range of from the softening temperature of the composition to about275 C.,

(c) a blowing agent which yields at least one mole of gas per mole ofblowing agent at a temperature within the range of from the softeningtemperature of the composition to about 275 C.,

said composition being heated to a temperature above the softening pointof said composition and sufiicient to release the gas from said blowingagent and to effect azido modification of said polymer, and

(2) reducing the pressure on the composition by opening the mold toallow free expansion at atmospheric pressure of the composition in alldirections to retain essentially the preformed shape.

2. The process of claim 1 wherein the thermoplastic polymer ispolypropylene.

3. The process of claim 1 wherein the thermoplastic polymer ispolyethylene.

4. The process of claim 1 wherein the thermoplastic polymer ispolystyrene.

5. The process of claim 1 wherein the thermoplastic polymer ispoly(vinyl chloride).

6. The process of claim 1 wherein the thermoplastic polymer is anethylene-propylene copolymer.

7. The process of claim 1 wherein the composition heated under pressurecomprises a mixture of thermoplastic polymer, an elastomeric polyolefin,an azido crosslinking agent and a blowing agent, said elastomericpolyolefin being selected from the group consisting of polyisobutylene,butyl rubber, ethylene-propylene copolymer rubber, andethylene-propylene-diene terpolyers.

8. The process of claim 7 wherein the elastomeric polyolefin is anethylene-propylene copolymer rubber.

9. The process of claim 7 wherein the elastomeric polyolefiu ispolyisobutylene.

10. The process of claim 2 wherein the foamed and cross-linkedpolypropylene is formed, while still at elevated temperature, into ashaped article.

11. The process of claim 10 wherein the foamed and cross-linkedpolypropylene is formed into a flat sheet by pressing between parallelplates.

12. The process of claim 10 wherein the foamed and cross-linkedpolypropylene is formed into a three-dimensional shaped article bypressing in a matched set mold.

13. A process for preparing cellular polypropylene which comprises 1)heating in a closed mold a preformed composition comprising p yp py e,

(b) an azido cross-linking agent which decomposes at a temperaturewithin the range of from the softening temperature of the composition toabout 275 C.,

(c) a blowing agent which yields at least one mole of gas per mole ofblowing agent at a temperature Within the range of from the softeningtemperature of the composition to about 275 C.,

said composition being heated to a temperature above the softening pointof said composition and suflicient to release the gas from said blowingagent and to effect azido modification of said polypropylene,

(2) reducing the pressure on the composition by open ing the mold toallow free expansion at atmospheric pressure of the composition in alldirections to retain essentially the preformed shape, and

(3) while still at elevated temperature reducing the pressure on thefoam to subatmospheric pressure to allow further expansion of the foam.

14. The process of claim 13 wherein the foamed and cross-linkedpolypropylene is formed into a shaped article.

15. A process for preparing cellular thermoplastic polymers whichcomprises (1) heating in a closed mold a preformed Particulatecomposition comprising (a) a thermoplastic polymer selected from thegroup consisting of polyethylene, polypropylene, ethylene-propylenecopolymers, ethylene-vinyl acetate copolymers, ethylene-alkyl acrylatecopolymers, polystyrne, acrylonitrile butadienestyrene terpolymers, andpolyvinyl chloride,

(13) alnazido cross-linking agent which decomposes at a temperaturewithin the range of from the softening temperature of the composition toabout 275 C.,

(c) a blowing agent which yields at least one mole of gas per mole ofblowing agent at a temperature within the range of from the softeningtemperature of the composition to about 275 C.,

said composition being heated to a temperature above the softening pointof said composition and sufiicient to release the gas from said blowingagent and to efiect azido modification of said polymer, and

(2) reducing the pressure on the composition. by opening the mold toallow free expansion at atmospheric pressure of the composition in alldirections to retain essentially the shaiPe of the mold.

16. The process of claim 15 wherein the thermoplastic polymer ispolypropylene.

17. The process of claim 15 wherein the thermoplastic polymer ispolyethylene.

18. The process of claim 15 wherein the thermoplastic polymer ispolystyrene.

19. The process of claim 15 wherein the thermoplastic polymer ispoly(vinyl chloride).

20. The process of claim 15 wherein the thermoplastic polymer is anethylene-propylene copolymer.

21. The process of claim 15 wherein the composition heated underpressure comprises a mixture of thermoplastic polymer, an elastomericpolyolefin, an azido crosslinking agent and a blowing agent, saidelastomeric polyolefin being selected from the group consisting ofpolyisobutylene, butyl rubber, ethylene-propylene copolymer rubber, andethylene-propylene-diene terpolymers.

22. The process of claim 21 wherein the elastomeric polyolefin is anethylene-propylene copolymer rubber.

23. The process of claim 21 wherein the elastomeric polyolefin ispolyisobutylene.

24. The process of claim 16 wherein the foamed and crosslinkedpolypropylene is formed, while still at elevated temperature, into ashaped article.

25. The process of claim 24 wherein the foamed and crosslinkedpolypropylene is formed into a flat sheet by pressing between parallelplates.

26. The process of claim 24 wherein the foamed and crosslinkedpolypropylene is formed into a three-dimensional shaped article bypressing in a matched set mold.

27. A process for preparing cellular polypropylene which comprises (1)heating in a closed mold a preformed particulate composition comprising(a) polypropylene,

(b) an azido crosslinking agent which decomposes at a temperature withinthe range of from the softening temperature of the composition to about275 C.,

(c) a blowing agent which yields at least one mole of gas per mole ofblowing agent at a temperature within the range of from the softeningternperature of the composition to about 275 C.,

said composition being heated to a temperature above the softening pointof said composition and sufiicient to release the gas from said blowingagent and to efiect azido modification of said polypropylene,

(2) reducing the pressure on the composition by opening the mold toallow free expansion at atmospheric pressure of the composition in alldirections to retain essentially the shape of the mold, and

(3) while still at elevated temperature reducing the pressure on thefoam to subatnzospheric pressure to allow further expansion of the foam.

28. The process of claim 27 wherein the foamed and crosslinkedpolypropylene is formed into a shaped article.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,271,498 1/1942 Overstreet 264-54 2,283,3165/1942 Cooper et al 264-55 2,297,018 9/1942 Oval-street 264-54 2,518,2498/1950 Ott 260-25 2,532,243 11/1950 Ott 260-25 2,678,293 5/1954 McMillanet a1. 260-2.5 2,768,407 10/1954 Lindemann 264-55 2,830,029 4/1958 Adams260-25 2,927,904 3/1960 Cooper 260-25 2,948,664 8/1960 Rubens et a1260-25 3,006,033 10/1961 Knox 264-54 3,017,371 11/1962 Hohenberg et a1.260-25 3,058,944 9/1962 Breslow et a1. 260-25 3,137,745 6/1964 Johnstone260-25 3,140,266 6/1964 Peticolas 260-25 3,250,730 5/1966 Palmer 260-253,250,731 5/1966 Buhl et a1 260-25 FOREIGN PATENTS 856,735 12/1960 GreatBritain.

MURRAY TILLMAN, Primary Examiner MORTON FOELAK, Assistant Examiner US.Cl. X.R.

P0405 UNITED STATES PATENT OFFICE 569 CERTIFICATE OF CORRECTION PatentNo. Reissue 26850 Dated April 7, 1970 Inventofls) David A. Palmer It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 2, formula reads R-E5O N j Should read:

R so N X 2 3 X Column formula reads R -0EM3 X Should read R c N Column3, line 55 this line does not appear in case Column 5, line 25, "foam"should read "form" @6111? AR SEALED (SEAL) Atteet:

WILLIAM E. W, .18 EdWRIdM-MJL Oomisaioner of Pam AttestingOffloer

